1. Technical Field
This invention relates to a method and apparatus for extruding or spinning synthetic fibers and relates more particularly to the production of a homogeneous web of polymeric fibers wherein at least some of the fibers in the web have different characteristics from other fibers in the web, and to unique products that can be produced from such fibers. Of particular importance is the production of a homogeneously mixed fibrous web of the type described wherein at least certain of the fibers are multi-component polymeric fibers, such as sheath/core bicomponent fibers and wherein, if desired, more than one multiple-component fiber may be uniformly dispersed throughout a web of fibers, with at least the sheath of such multiple-component fibers being formed of different polymeric materials.
This invention is also concerned with unique fibrous products having diverse applications, and particularly to such products when made using the advanced homogeneous mixed fiber technology referred to above.
This invention also relates to a heat and moisture exchanger and more particularly to a gas-permeable element, preferably comprising a fibrous media which may be made by the improved mixed fiber technology discussed above and which is adapted to be warmed and to trap moisture from a patient's breath during exhalation and to be cooled and to release the trapped moisture for return to the patient during inspiration, to thereby conserve the humidity and body heat of the patient's respiratory tract during treatment of the patient requiring communication of the patient with an extracorporeal source of a gas through an artificial airway. The heat and moisture exchanger of this respiratory tact may cause post-operative patient shivering and require unnecessary patient reheating during recovery.
Another complication resulting from the need to connect a patient to an extracorporeal source of gas through an artificial airway is the possibility of infecting the patient with bacterial, viral or other contaminants present in the inspired gas. Similarly, contaminants passing to the environment through the artificial airway can pollute the atmosphere. These problems are particularly important when treating infected or immno-compromised patients, or in the intensive care unit where both the patient being treated and other patients in the area are likely to be especially sensitive to the airborne transmission of pathogenic organisms.
2. Discussion of the Prior Art
Various prior art techniques are known for the production of polymeric fibers, including monocomponent fibers and multiple-component fibers of various configurations. Among such multiple-component fibers, bicomponent fibers comprising a core of one polymer and a coating or sheath of a different polymer are particularly desirable for many applications.
For example, in my prior U.S. Pat. No. 5,509,430 issued Apr. 23, 1996, the subject matter of which is incorporated herein in its entirety by reference, unique polymeric bicomponent fibers comprising a core of a low cost, high strength, thermoplastic polymer, preferably polypropylene, and a bondable sheath of a material which may be cellulose acetate, ethylene-vinyl acetate copolymer, polyvinyl alcohol, or ethylene-vinyl alcohol copolymer are disclosed for use particularly in the production of tobacco smoke filters. The bicomponent fibers produced according to the techniques of the '430 patent may be melt blown to produce very fine fibers, on the order of about 10 microns or less in diameter, in order to obtain enhanced filtration. Such products are shown to have improved tobacco smoke filtration efficiency, acceptable taste, and can be produced at a substantially lower cost than conventional tobacco smoke filters formed from fibers consisting entirely of cellulose acetate.
In my subsequent U.S. Pat. No. 5,607,766 issued Mar. 4, 1997, U.S. Pat. No. 5,620,641 issued Apr. 15, 1997, and U.S. Pat. No. 5,633,082 issued May 27, 1997, the subject matters of which are also incorporated herein in their entireties by reference, unique melt blown bicomponent fibers comprising a core of a thermoplastic material covered by a sheath of polyethylene terephthalate and methods of making same are disclosed as particularly useful in the production of elongated, highly porous elements having numerous applications. For example, such products are useful as wick reservoir elements for marking and writing instruments, that is, materials designed to take up a liquid and later controllably release the same as in an ink reservoir. Additionally, because of their high capillarity, such materials function effectively in the production of simple wicks for transferring liquid from one place to another, such as in the production of the fibrous nibs found in certain marking and writing instruments. Wicks of this sort are also useful in diverse medical applications, for example, the transport of bodily fluid by capillary action to a test site in a diagnostic device.
Products made from the bicomponent fibers of the '766, '641 and '082 patents are also shown to be useful as absorption reservoirs, i.e., as a membrane to take up and simply hold the liquid as in a diaper or an incontinence pad. Absorption reservoirs are also useful in medical applications. For example, a layer or pad of such material may be used in an enzyme immunoassay test device where they will draw a bodily fluid through the fine pores of a thin membrane coated, for example, with monoclonal antibodies that interact with antigens in the bodily fluid which is pulled through the membrane and then held in the absorption reservoir. Such materials are also suggested, with the possible addition of a smoke-modifying or taste-modifying material, for use in tobacco smoke filters.
Polymeric fibers, in general, may be produced by a number of common techniques, oftentimes dictated by the polymer itself or the desired properties and applications for the resultant fibers. Among such techniques, are conventional melt spinning processes wherein molten polymer is pumped under pressure to a spinning head and extruded from spinneret orifices into a multiplicity of continuous fibers. Melt spinning is only available for polymers having a melting point temperature less than its decomposition point temperature, such as nylon, polypropylene and the like whereby the polymer material can be melted and extruded to fiber form without decomposing. Other polymers, such as the acrylics, cannot be melted without blackening and decomposing. Such polymers can be dissolved in a suitable solvent (e.g., acetate in acetone) of typically 20% polymer and 80%/solvent. In a wet solution spinning process, the solution is pumped, at room temperature, through the spinneret which is submerged in a bath of liquid (e.g. water) in which the solvent is soluble to solidify the polymeric fibers. It is also possible to dry spin the fibers into hot air, rather than a liquid bath, to evaporate the solvent and form a skin that coagulates. Other common spinning techniques are well known and do not form a critical part of the instant inventive concepts.
After spinning, the fibers are commonly attenuated by withdrawing them from the spinning device at a speed faster than the extrusion speed, thereby producing fibers which are finer and, depending upon the polymer, possibly, more crystalline in nature and, thereby, stronger. The fibers may be attenuated by taking them up on rotating nip rolls or by melt blowing the fibers, that is, contacting the fibers as they emanate from the spinneret orifices with a fluid, such as air, under pressure to draw the same into fine fibers, commonly collected as an entangled web of fibers on a continuously moving surface, such as a conveyor belt or a drum surface, for subsequent processing.
As described in my aforementioned patents, the extruded fibrous web may be gathered into a sheet form which may be pleated to increase the surface area for certain filtering applications. Alternatively, the web of fibers may be gathered together and passed through forming stations, such as steam treating and cooling stations, which may bond the fibers at their points of contact to form a continuous rod-like porous element defining a tortuous path for passage of a fluid material therethrough.
While earlier techniques and equipment for spinning fibers have commonly extruded one or more polymer materials directly through an array of spinneret orifices to produce a web of monocomponent fibers or a web of multiple-component fibers, recent development incorporate a pack of disposable distribution or spin plates juxtaposed to each other, with distribution paths being etched into upstream and/or downstream surfaces of the plates to direct streams of one or more polymer materials to and through spinneret orifices at the distal end of the spinning system. These techniques are embodied, for example, in Hills U.S. Pat. No. 5,162,074 issued Nov. 10, 1992, the subject matter of which is incorporated herein in its entirely by reference, and provide a reasonably inexpensive way to manufacture highly sophisticated spinning equipment and to produce a high density of continuous fibers formed of more than one polymeric material. Hills recognizes the production of multiple-component fibers, such as bicomponent fibers, wherein the components adhere to each other in a durable fashion, or, alternatively, are poorly adhering so that the components may be split apart to increase the effective fiber yield from each spinneret opening and to produce finer fibers from the individual components.
Although Hills and others provide relatively inexpensive, even disposable, distribution plates capable of spinning a high density of identical fibers, which may include separable segments of different polymeric materials, and the production of a web of mixed fibers, i.e., fibers having different physical and/or chemical characteristics, is broadly referred to in the literature, to my knowledge the prior art fails to recognize the advantages of directly spinning a homogeneous or uniform mixture of fibers from a spinning device, wherein the fibers extruded from certain of the spinneret orifices in the same element have different characteristics from the fibers extruded from other spinneret orifices in that element. Moreover, the techniques and equipment currently commercially available are not adapted to produce such a homogeneous web of mixed fibers, most especially, a uniformly distributed mixture of monocomponent and multiple-component fibers, or even a uniform mixture of different multiple-component fibers, e.g., where adjacent fibers in the web have different polymeric coatings such as alternating bicomponent fibers having a common core-forming polymer and different sheath-forming polymers.
Although fibrous products, including the unique melt-blown bicomponent fibers of my '430, '766, '641 and '082 patents discussed above, have significant commercial applications, the functional properties of the available products are limited by the inability of prior art technology to produce uniform and consistent webs of mixed fibers of differing chemical and/or physical characteristics. To the extent that the prior art is capable of producing mixed fibrous webs, the apparatus and techniques for doing so are generally inadequate for commercial application and/or are unable to provide reproducible, highly homogeneous, mixtures of diverse fibers from the same set of spinneret orifices.
With an improved ability to produce mixed fiber webs of substantially complete uniformity, improved functional properties can be afforded in a variety of fibrous products, whether they are intended to for use as high efficiency filters such as are required in electric dust collection devices and power plants, coalescent-type filters such as those used to separate water from aviation fuel, wicking products such as may be used for ink transfer in marking and writing instruments or as medical wicks, or in similar liquid holding and transferring applications, or in diverse other fields.
With respect to a particular application of the improved technology of this invention, that is, in the production of heat and moisture exchangers and high efficiency particulate air filters for use in a breathing circuit requiring an artificial airway, various prior art devices are commercially available. Oftentimes, however, separate devices are necessary to conserve the humidity and body heat of the patient's respiratory tract and to filter undesirable constituents from a gas being inhaled by the patient, or from the patient's breath exhaled during such treatments. Although some devices are available which incorporate media capable of performing all of these functions, it is not uncommon in such devices for particular properties to be compromised in order that other properties can be enhanced. The availability of a device capable of maximizing both heat and moisture exchange and filtration in an economic manner would be most desirable.
Early attempts to humidify a patient's respiratory tract and thereby reduce heat loss during short or long-term mechanical ventilation or the like, utilized electrically heated, water-filled humidifiers to add water vapor to the airway. This approach produced almost as many problems as it solved. The water level and temperature of the water vapor required constant monitoring. Further, particular difficulty was experienced in controlling the delivery of the small volumes of moisture needed for children or infants. Condensation of the water vapor could plug the small airways and, in extreme situations, even cause drowning. Additionally, the development of deposits in the humidifier reservoir often contaminated the moisture, thereby damaging the equipment and possibly harming the patient. The presence of such contaminants simply increased the need for effective filtration.
More recently, regenerative humidifiers or “artificial noses” have been developed as safe and effective alternatives to overcome many of the foregoing problems with heated water bath humidifiers. Such units are commonly referred to as heat and moisture exchangers (HMEs) because they function much in the same way as the patient's natural resources, that is, they capture the moisture and heat as the patient exhales and return them to the patient during the next breath.
HMEs are passive, requiring no outside source of moisture or power. They are placed in line with the artificial airway and are provided with a media producing a large surface area for the exchange of heat and moisture The HME media is warmed as humidity in the patient's breath condenses during exhalation, is cooled during inhalation as it gives up heat and moisture vapor to the inspired gases, and the process is repeated as the patient breathes in and out.
Attempts have been made to increase the hygroscopicity of the HME media to thereby directly absorb moisture from exhaled gases, whereby the media retains more moisture than would have been collected from condensation alone to thereby improve the HME output. Further, since the moisture held by the hygroscopic media is absorbed and not condensed, vaporative cooling of the HME is limited when this moisture is released during inhalation.
While the concept is technically sound, the particular hygroscopic materials commercially available are either inadequate or undesirable for use as HME media. Additives such as salts, e.g., lithium chloride, or glycerin provide advantageous hygroscopicity to HME media, but can contaminate and even interact with gases passing through such media during inspiration by the patient. Provision of an HME media capable of attracting and holding additional moisture from a patient's breath during exhalation without the need for extraneous chemicals is important to the safe and effective operation of an HME in auxiliary breathing equipment.
A number of criteria are particularly important in the design of an HME for medical applications. Low thermal conductivity of the heat and moisture exchange media increases the temperature differential across the HME, improving its efficiency. A low pressure drop across the HME is essential to minimize effort during normal breathing or mechanical ventilation. An HME must also be relatively lightweight since it is to be supported at a tracheotomy, endotracheal or nasotracheal site in most applications. The HME media should be disposable or easily sterilized to minimize costs in maintaining the breathing circuit. Finally, the HME media should be effective without the need for chemical additives that could affect the treated gases, and the media should not release any particulate matter, thereby protecting the patient and the environment as well as the equipment with which the HME is associated against contamination.
In summary, the HME must efficiently, inexpensively and safely provide adequate heat and moisture, preferably, to enable a single unit to effectively conserve the humidity and body heat of the patient's respiratory tract and, if possible, concomitantly filter gases passing therethrough to remove particulate contaminants, thereby avoiding the need for redundant units.